Hyperion, or Saturn VII, is the eighth moon of Saturn to be discovered, and the largest irregularly shaped body in the Solar system. (Proteus, moon of Neptune, is heavier but has a more nearly spheroid shape.) It is also highly porous, having the appearance of a sponge, and displays a chaotic rotation.

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Discovery and naming

William Cranch Bond and George Phillips Bond first noticed Hyperion as a faint object in Saturn's ring plane, between the moons Titan and Iapetus. In the course of several days, the Bonds saw this object again, and it was clearly keeping station with Saturn at the time of Saturn's apparent retrograde motion. They determined its sidereal month with remarkable accuracy and reported their findings in the Monthly Notices.[1]

Independently of them, William Lassell first observed Hyperion on September 18, 1848. He was searching for Iapetus and found two objects, Iapetus and another, much fainter object. The next day, the faint object had appeared to approach Saturn, while the other object had not. Lassell was satisfied that the brighter object was Iapetus and determined that he had discovered a new object.[9]

Sir John Herschel had earlier suggested a naming convention for the seven satellites of Saturn already discovered. Lassell and the Bonds independently selected Hyperion as the name of the new object, in accordance with that convention.

Orbital characteristics

Hyperion is in a slightly eccentric orbit around Saturn and participates in a 4:3 orbital resonance with Titan. Its sidereal day is about 21.28 earth days.[8][10] Many astronomers believe that the 4:3 resonance with Titan is responsible for the eccentricity of Hyperion's orbit.[8]

Rotational characteristics

Hyperion's rotation is chaotic and nearly unpredictable.[11] Astronomers cite a number of factors, including the 4:3 orbital resonance with Titan, to account for this.[8][12][13]

Physical characteristics

Hyperion has an irregular shape, with dimensions 185 x 140 x 113 km.[5] In fact it is the largest irregularly shaped body in the solar system. Proteus, moon of Neptune, is often cited as a larger irregular body, but in fact Proteus is not irregular and is almost spherical.[12] Hyperion is also highly porous, with a density only 57% of that of water.[5]

Surface

Hyperion in color enhancement

Hyperion has craters that cover its surface completely, making it spongiform in appearance.[14] Those craters are remarkably well preserved, primarily on account of Hyperion's low density. The largest crater on Hyperion has a diameter of 120 km and a depth of 10 km.[13]

Hyperion is one of the least reflective of Saturn's moons, with an albedo of 30%. Many astronomers believe that Hyperion is covered with dark material that comes from Phoebe, an outer moon.[6] Recent data from the Cassini probe indicates that this material consists of hydrocarbons.[15]

The favorite theory of the origin of Hyperion is that it is a fragment of a larger body that suffered a collision.[12][13] At least one astronomer speculated that debris from this collision contributed to the dark material that covers the Cassini Region of Iapetus.[16]

Observation and Exploration

Hyperion was under observation for centuries, but only with modern rocket probes has detailed examination been possible. The first such probe to make rendezvous with Hyperion was Voyager 2, on August 24, 1981. The best examination of Hyperion to date was by the Cassini spacecraft, which made rendezvous with Hyperion on September 26, 2005 and took the remarkable images of its spongiform surface.